If you are using heat engine that is shipped with UCD then you can only install on Redhat. Where as if you are extending the heat engine (i.e installing engine which is not shipped withe the product) then you can install it on ubuntu

"You must install the engine on a host that runs a specific version of Red Hat Enterprise Linux (RHEL).
In version 6.2.1.1 and later, the IBM® UrbanCode™ Deploy installer can install engines for the blueprint design server only on RHEL version 7. No other operating system is supported, including RHEL version 6 or earlier.
In versions before 6.2.1.1, the IBM UrbanCode Deploy installer can install engines for the blueprint design server only on RHEL version 6. No other operating system is supported, including RHEL version 7.
To use a different operating system, you can install an OpenStack Heat engine manually on that operating system and then extend that engine for use with the blueprint design server; see Extending Heat orchestration engines."